Diamine hole transport agent and organic electroluminescent device containing same

ABSTRACT

A novel diamine compound of formula (I) of the present invention having a high glass transition temperature is useful as an improved hole transport agent, and an organic electroluminescent device containing the diamine compound of formula (I) has good thermal and mechanical stabilities, and prolonged lifetime.

FIELD OF THE INVENTION

The present invention relates to a novel diamine hole transport agent,and, more particularly, to a novel diamine compound having an improvedhole transport capability and a high glass transition temperature, andan organic electroluminescent device containing same.

BACKGROUND OF THE INVENTION

Generally, an organic electroluminescent device has a laminatedstructure comprising a transparent electrode layer, a metallic electrodelayer, and an organic interlayer including an organic luminescent layerarranged between the two electrodes. The organic interlayer comprisingan organic luminescent layer may further include a hole transport layersandwiched between the transparent electrode layer and the organicluminescent layer, and may still further include an electron transportlayer inserted between the metallic electrode layer and the organicluminescent layer, often in a multilayer configuration to increaseluminous efficiency.

Hitherto, aromatic tertiary amines such astetraphenylbiphenyldiamine(TPD), N-phenylcarbazole, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(NPB) have been used informing a hole transport layer.

However, due to the fact that the above-mentioned compounds have lowglass transition temperatures ranging from 60 to 95° C.,electroluminescent devices containing such hole transport agents havelow heat stability, bad resistance to external impact, and shortlifetime under hard environment.

Therefore, there has existed a need to develop a novel compound having ahigh glass transition temperature for use in an electroluminescentdevice with improved hole transport capability.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea novel compound having an improved hole transport capability and a highglass transition temperature, and an organic electroluminescent devicecontaining same.

In accordance with one aspect of the present invention, there isprovided a diamine compound of formula (I):

wherein, Ar₁ is a phenyl or biphenyl group, and Ar₂, is a phenylene or

group, Ar₁ and Ar₂ being optionally fused to form, together with thenitrogen atom attached thereto, substituted carbazole derivative.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description thereof, when taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of an organic electroluminescent deviceprepared in accordance with one embodiment of the present invention;

FIG. 2 shows an infrared spectrum of the compound of formula (Ia)prepared in Example 1;

FIG. 3 reproduces an absorption spectrum of the compound of formula (Ib)prepared in Example 2; and

FIG. 4, a luminescent spectrum of a thin layer made thereof;

FIGS. 5 and 6 exhibit absorption and infrared spectra of the compound offormula (Ic) prepared in Example 3, respectively; and

FIG. 7, a luminescent spectrum of a thin layer made thereof;

FIG. 8 shows an electroluminous spectrum of the electroluminescentdevice prepared in Example 4;

FIG. 9, variations of the current density(A/m²) () andbrightness(cd/m²) (∘) as function of the applied voltage(V); and

FIG. 10, the change in the luminous efficiency(lm/W) with currentdensity(A/m²); and

FIGS. 11 and 12 depict an electroluminescent spectrum of theelectroluminescent device prepared in Example 5, and variations of thecurrent density(A/m²) (12-1) and brightness(cd/m²) (12-2) with theapplied voltage(V), respectively.

DETAILED DESCRIPTION OF THE INVENTION

a diamine derivative of formula (I) of the present invention containstwo groups of primary amine as well as two groups of tertiary amine, andrepresentative examples thereof have the structures of formulae (Ia) to(Ic):

The above compounds of formulae (Ia) to (Ic) may be obtained bynitrating the corresponding compounds of formulae (IIa) to (IIc) to formcompounds of formulae (IIIa) to (IIIc) and reducing the compounds,respectively:

The starting compounds of formulae (IIa) to (IIc) may be synthesized bya conventional method, e.g., by using a Cu catalyst (see U.S. Pat. No.4,265,990). The nitration of each of the compounds of formulae (IIa) to(IIc) can be conducted using at least a 2 molar equivalent amount ofnitric acid in acetic acid or a mixture of acetic acid and sulfuricacid(50:50 to 95:5), at a temperature ranging from 10 to 90° C.,preferably 50 to 70° C. for one hour or more. The resulting nitrocompound is converted to the corresponding amine compound using areducing agent which may be employed in an amount ranging from 1.5 to 5molar equivalent amount based on the amount of the nitro-group.Representative examples of the reducing agent include Fe, sulfides andhydrogen, among which NaSH is preferred. The reduction can be carriedout in an alcohol solvent such as ethanol, propanol and butanol, or in amixture of alcohol and water(50:50 to 95:5), at a temperature rangingfrom 60 to 150° C., preferably 80 to 120° C.

Alternatively, the compounds of formulae (Ia) and (Ib) may be preparedby coupling the compound of formula (IV) with the compounds of formulae(V) and (VI) to form compounds of formulae (IIIa) and (IIIb),respectively and reducing the compounds:

The bromonitrobiphenyl of formula (VI) may be formed by nitratingbromobiphenyl. The coupling reaction may be conducted in a solvent suchas xylene, toluene and dimethylsulfoxide at a temperature ranging from80 to 180° C., in the presence of a catalyst such as Cu, CuCl andPd/phosphine.

In addition, the compound of formula (Ib) may be prepared by reactingthe secondary amine compound of formula (VII) with a compound of formula(VIII) to form the compound of formulae (IIIb), and reducing thecompound:

wherein, X is iodine or bromine.

The secondary amine compound of formula (VII) may be synthesized byreacting aniline with the compound of formula (VI). The reaction betweenthe compounds of formulae (VII) and (VIII) may be carried out in asolvent such as xylene, toluene and dimethylsulfoxide at a temperatureranging from 80 to 180° C., in the presence of a catalyst such as Cu,CuCl and Pd/phosphine.

The inventive diamine derivative of formula (I), e.g., compounds offormulae (Ia), (Ib) and (Ic), prepared in accordance with theabove-described method has a high glass transition temperature of 100°C. or higher and exhibits improved hole transporting characteristics.Accordingly, the inventive diamine may fabricate a hole transport layeras is, and, further, be used to form a stable polymer(e.g., polyimide,polyamide and polyurea) hole transport layer.

The organic electroluminescent device of the present invention comprisesthe inventive diamine derivative of formula (I) as a hole transportagent. The inventive electroluminescent device comprises an organicinterlayer which may be of a multi-layer structure containing a holetransport layer and an organic luminescent layer. The organic interlayermay further include an electron transport layer, inserted between themetallic electrode layer and the organic luminescent layer.

An example of the organic electroluminescent device of the presentinvention contains a triple-layered organic interlayer as shown in FIG.1. The device consists of a transparent substrate(1), a transparentelectrode layer(2), a hole transport layer(3), an organic luminescentlayer(4), an electron transport layer(5) and a metallic electrodelayer(6). An electric power supply(7) may be either alternatingcurrent(AC) or direct current(DC).

The hole transport layer of the present invention may be formed by aconventional method which may be a wet process, e.g., a spin-coating,doctor-blading, roll-printing and screen-printing method, or a dryprocess, e.g., a vapor deposition, vacuum thermal deposition, sputteringand electron beam deposition method. The thickness of the hole transportlayer may range from 5 to 300 nm.

Exemplary electroluminescent materials which may be used in the presentinvention include tris (8-hydroxyquinolinolato)aluminum (Alq₃),1,1,4,4-tetraphenyl-1,3-butadiene(TB), oligophenylenevinylenederivatives,4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran(DCM),1,4-distyrylbenzene, anthracene, tetracene, pentracene, coronene,perylene, pyrene, bis (8-quinolinolato) zinc(II),9,10-diphenylanthracene, tris(4,4,4-trifluoro-1-(2-thienyl)-1,3-butandiono)-1,10-phenanthrolineeuropium(III), tris(2,4-pentadiono)-1,10-phenanthroline terbium(III),tris(4,4,4-trifluoro-1-(2-thienyl)-1,3-butandiono)-1,10-phenanthrolinedysprosium(III), and a mixture thereof. The thickness of the organicluminescent layer may range from 5 to 300 nm.

Exemplary electron transport agents which may be used in the presentinvention include Alq₃, LiF, and oxadiazole derivatives such as2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(B-PBD),3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole(TAZ),2,5-bis(1-naphtholyl)-1,3,4-oxadiazole(BND), and a mixture thereof. Theelectron transport layer may be formed on the organic luminescent layerin a thickness ranging from 5 to 300 nm.

The organic electroluminescent device of the present invention has goodthermal and mechanical properties, improved stability and prolongedlifetime.

The present invention is further described and illustrated in Example,which is, however, not intended to limit the scope of the presentinvention.

EXAMPLE 1 Preparation of Compound of Formula (Ia)

(Step 1) Preparation of Compound of Formula (IIIa)

3.364 g of N,N′-diphenylbenzidine (formula (IV)), 5.975 g ofiodonitrobenzene (formula (V)), 110 mg of Pd(OAc)₂ and 220 mg oftri-o-tolyl phosphine were added to 30 ml of refined toluene, and 2.5 gof sodium-t-butoxide was added thereto under a nitrogen atmosphere. Themixture was heated at 65° C. for 4 hours. Then, the reaction mixture wascooled to room temperature and 100 ml of chloroform was added. Theresulting solution was filtered through a silica gel column and thesolvent was removed from the filtrate under a reduced pressure to obtaina residue, which was recrystallized from a mixture of acetone andmethanol to obtain 4.63 g (yield: 80%) of the title compound (formula(IIIa)) as a yellow solid.

¹H-NMR(DMSO-D₆): δ 8.09(d,4H), 7.74(d,4H), 7.47(t,4H), 7.20˜7.35(m,10H),6.87(d,4H).

(Step 2) Preparation of Compound of Formula (Ia)

0.578 g of the compound (formula (IIIa)) obtained in (Step 1) and 0.32 gof 70 % NaSH were added to a mixture of 5 ml of n-butanol and 5 ml oftoluene, and the mixture was refluxed for 5 hours. The solvent wasremoved from the reaction mixture under a reduced pressure and theresidue was treated with a mixture of 15 ml of water and 15 ml ofchloroform. Subsequently, the mixture was extracted with chloroform (150ml×3), the organic extracts were combined, and chloroform was removedtherefrom under a reduced pressure to obtain 0.43 g (yield: 83%) of thetitle compound (formula (Ia)).

¹H-NMR(DMSO-D₆): δ 7.43(d,4H), 7.21(t,4H), 6.58 ˜6.95(m,14H),6.57(d,4H).

FIG. 2 shows an infrared spectrum of the compound (formula (Ia)) thusobtained.

EXAMPLE 2 Preparation of Compound of Formula (Ib)

(Step 1) Preparation of Compound of Formula (VI)

4.66 g of bromobiphenyl was added to a mixture of 6 g of sulfuric acidand 35 g of acetic acid, and 2.2 g of 60% HNO₃ was added slowly theretoat 25° C. for 5 min. The mixture was heated at 750 ° C. for 4 hours.Then, the reaction mixture was cooled to room temperature and theprecipitates were filtered, washed with water and dried to obtain 4.40 g(yield: 80%) of the title compound (formula (VI)) as a white solid.

¹ H-NMR(CDC₃ 13): δ 8.31 (d,4H), 7.71(d,4H), 7.63(d,4H), 7.49(d,4H).

(Step 2) Preparation of Compound of Formula (IIIb)

2.92 g of the compound (formula (VI)) obtained in (Step 1), 1.68 g ofN,N′-diphenylbenzidine (formula (IV)), 55 mg of Pd(OAc)₂ and 220 mg oftri-o-tolyl phosphine were added to 10 ml of refined xylene, and 1.25 gof sodium-t-butoxide was added thereto under a nitrogen atmosphere. Themixture was heated at 120° C. for 3 hours. Then, the reaction mixturewas cooled to room temperature and 50 ml of chloroform was addedthereto. The resulting solution was filtered and the solvent was removedfrom the filtrate under a reduced pressure and the residue was purifiedby column chromatography (eluent; chloroform:n-hexane=1.1) to obtain2.63 g (yield: 72%) of the title compound (formula (IIIb)).

¹H-NMR(DMSO-D₆):δ 8.26 (d,4H), 7.92(d,4H), 7.74(d,4H), 7.63(d,4H),7.38(t,4H), 7.07˜7.15(m,14H).

(Step 3) Preparation of Compound of Formula (Ib)

1.46 g of the compound (formula (IIIb)) obtained in (Step 2) and 0.64 gof 70% NaSH were added to a mixture of 25 ml of n-butylalcohol and 2 mlof water, and the mixture was heated at 110° C. for 24 hours. Thesolvent was distilled off under a reduced pressure. The resultingresidue was treated with 10 ml of water, filtered, and dried to obtain1.16 g (yield: 91%) of the title compound (formula (Ib)).

¹H-NMR(DMSO-D₆) δ7.55(d,4H), 7.48(d,4H), 7.28 ˜7.31(m,8H), 7.02˜7.07(m,14H), 6.61(d,4H).

An absorption spectrum of the compound (formula (Ib)) thus obtained isshown in FIG. 3; and a luminescent spectrum of a thin layer thereofprepared by vapor deposition, in FIG. 4.

EXAMPLE 3 Preparation of Compound of Formula (Ic)

(Step 1) Preparation of Compound of Formula (IIc)

7.023 g of carbazole, 8.12 g of 4,4′-diodobiphenyl, 5.528 g of K₂CO₃ and0.396 g of CuCl were added to 20 ml of DMSO, under a nitrogenatmosphere. The mixture was heated at 140° C. for 1 hour, and then at170° C. for 3 hours. Then, the reaction mixture was cooled to below 50°C. and 100 ml of water was added thereto. The resulting solution wasstirred for 30 min. and filtered. The filtrate was dried to produce asolid, which was dissolved in 100 ml of methanol. The solution wasrefluxed for 30 min. and filtered. The resulting filtrate was dried toobtain 9.49 g (yield: 98%) of the title compound (formula (IIc)) as alight gray solid.

¹ H-NMR(CDC 1₃):δ 8.18 (d, 4H), 7.92(d,4H), 7.72(d,4H), 7.52(d,4H),7.45(t,4H), 7.32(t,4H)

(Step 2) Preparation of Compound of Formula (IIIC)

4.855 g of the compound (formula (IIc)) obtained in (Step 1) and 2.31 gof 60% HNO₃ were added to 100 ml of acetic acid. The mixture was heatedat 65 ° C. for 2 hours. Then, the reaction mixture was cooled to roomtemperature, and the solid formed was filtered, washed with water anddried to obtain 5.46 g (yield: 95%) of the title compound (formula(IIIc)) as a light yellow.

¹H-NMR(CDCl₃): δ 9.12(s,2H), 8.38(d,2H), 8.26(d,2H), 7.97(d,4H),7.73(d,4H), 7.48˜7.53(m,8H)

(Step 3) Preparation of Compound of Formula (Ic)

0.575 g of the compound (formula (IIIc)) obtained in (Step 2) and 0.64 gof 70% NaSH were added to 20 ml of n-butanol, and the mixture wasrefluxed for 3 hours. The solvent was removed from the reaction mixtureunder a reduced pressure. The solid thus obtained was treated with 20 mlof water, filtered, dried and purified by column chromatography (eluent;chloroform:n-hexane=1:1) to obtain 0.42 g (yield: 82%) of the titlecompound (formula (Ic)).

¹H-NMR(DMSO-D₆): δ 8.04(d,4H), 7.73(d,4H), 7.16˜7.47(m,12H), 6.81(d,2H).

Absorption and infrared spectra of the compound (formula (Ic)) thusobtained are shown in FIGS. 5 and 6, respectively; and a luminescentspectrum of a thin layer thereof prepared by vapor deposition, in FIG.7.

EXAMPLE 4 Preparation of an Organic Electroluminescent Device—(1)

Indium-tin-oxide(ITO) was coated on a glass substrate to form atransparent anode layer. The compound of formula (Ib) obtained inExample 2 was vapor deposited on the transparent anode layer to athickness of 30 nm at a rate of 0.03 nm/sec at 5×10⁻⁶ torr to form ahole transport layer. Then, Alq₃ was vapor deposited on the holetransport layer to a thickness of 20 nm at a rate of 0.03 nm/sec at5×10⁻⁶ torr to form a luminescent/electron transport layer.Subsequently, aluminum was vapor deposited on the luminescent/electrontransport layer to a thickness of 300 nm at a rate of 1 nm/sec at 2×10⁻⁵torr to form a cathode layer.

The luminescence characteristics of the organic electroluminescentdevice thus obtained are shown in FIGS. 8, 9 and 10.

FIG. 8 reproduces an electroluminous spectrum of the electroluminescentdevice.

FIG. 9 illustrates the variations of the current density(A/m²) () andbrightness(cd/m²) (∘) of the electroluminescent device as function ofthe applied voltage (V). The current injection starts at about 2V, turnon voltage is about 8 to 9V, and the brightness is 500 cd/m² at 12V.

FIG. 10 exhibits the change in the luminous efficiency(lm/W) of theelectroluminescent device with current density(A/m²). The luminousefficiency is steady at 1.2 lm/W at a current density of 300 A/m² andbeyond.

EXAMPLE 5 Preparation of an Organic Electroluminescent Device—(2)

Indium-tin-oxide(ITO) was coated on a glass substrate to form atransparent anode layer. The compound of formula (Ic) obtained inExample 3 was vapor deposited on the transparent anode layer to athickness of 50 nm at a rate of 0.03 nm/sec at 5×10⁻⁶ torr to form ahole transport layer. Then, Alq₃ was vapor deposited on the holetransport layer to a thickness of 20 nm at a rate of 0.03 nm/sec at5×10⁻⁶ torr to form a luminescent/electron transport layer.Subsequently, aluminum was vapor deposited on the luminescent/electrontransport layer to a thickness of 300 nm at a rate of 1 nm/sec at 2×10⁻⁵torr to form a cathode layer.

The luminescence characteristics of the organic electroluminescentdevice thus obtained are shown in FIGS. 11 and 12.

FIG. 11 depicts an electroluminescent spectrum of the electroluminescentdevice.

FIG. 12 shows the variations of the current density(A/m²) (12-1) andbrightness(cd/m²) (12-2) of the above electroluminescent device asfunction of the applied voltage (V). The current injection starts atabout 6V, turn on voltage is about 7 to 8V, and the brightness is 200cd/m2 at 16V.

As can be seen from the above result, an organic electroluminescentdevice containing the novel hole transport agent of the presentinvention has an improved luminous efficiency. Also, the inventiveelectroluminescent device has an improved stability and prolongedlifetime due to the high glass transition temperature of the inventivehole transport agent. Therefore, the inventive electroluminescent devicemay be advantageously applied to solar cells, photorefraction thin layerdevices, organic photoconductors, field effect transistors, photodiodesand the like.

While the invention has been described with respect to the specificembodiments, it should be recognized that various modifications andchanges may be made by those skilled in the art to the invention whichalso fall within the scope of the invention as defined by the appendedclaims.

What is claimed is:
 1. The diamine compound of formula (Ic):


2. An organic electroluminescent device comprising a hole transportlayer containing the diamine compound of claim 1.